Do we have a tendency to turn around an obstacle in one direction more than in the other? And if the answer is yes, what are the factors responsible for such an asymmetry?
A bias in spontaneous turning has been observed in several animal species, at the individual or group level. There has been no consensus so far on the existence of such a bias in humans, probably due to lack of control of the factors likely to modulate this bias. To check whether such a bias can be reliably observed in humans, we conducted a first experiment in which we asked thirteen young adults to run around a circle in an empty, symmetrical space, as a function of starting position (from the left, the center, or the right), and gaze direction (to one of five targets going from left to right). A clear significant overall tendency to turn counterclockwise across all conditions was observed. This was particularly striking when the participants were required to start from the center position and look straight ahead before starting, with more than 80% of the participants turning counterclockwise in this perfectly symmetrical condition. Starting position and gaze and head direction modulated the bias, without masking the counterclockwise tendency. What can explain such a tendency?
Many underlying factors could be responsible for this counterclockwise tendency, since many different systems take part in locomotion (vestibular, visual, motor, space mental representation), and it is not easy to tear apart the influence of each of them. We hypothesized that the tendency to turn around the circle in a counterclockwise direction may come from three main origins: motor-peripheral, visuo-spatial, and mental space representation.
We thus designed a second experiment to see whether alternating steps without vision would be sufficient to induce a directional tendency. In this experiment, we asked twenty five young adults, blindfolded, to step on the spot while holding a rotating vertical roll bar fixed on the wall. Four experimental conditions (i.e., normal, with an imposed pace, dual-task, or with the neck bent) were tested. All participants deviated towards one side or the other in all conditions. Adding an attentional load or imposing a particular pace did not change the amount of deviation. For three conditions (normal, with an imposed pace and dual-task), the deviation towards one side was not significantly larger than towards the other side at the group level. Only in the bent-neck condition was the deviation towards the left significantly larger than the deviation towards the right at the group level. We concluded that, unless the vestibular feedback is perturbed, there is no evidence of asymmetry at the group level during stepping. Since we found no relationship between lateral preferences (hand, foot, and eye) and the side of deviation, our results go clearly against the idea that sensori-motor lateral preferences are among the factors underlying such deviations.
In a third experiment we tried to dissociate the influence of visual attention and mental space representation from that of motor factors on directional tendencies. For that purpose, we compared directional choices in three conditions: in one condition, the subjects were required to mark the direction chosen to turn around a circle drawn on a sheet of paper. In a second condition, they were asked to indicate verbally what direction they imagined the person seen on the screen would choose to turn around the circle. Finally, in the third condition, they had to move a cursor in a virtual environment with the help of the mouse, when turning around obstacles situated in the middle of the virtual rooms. A directional tendency was clearly observed only in the virtual environment condition, and it consisted in a tendency to go around by the right side of the obstacle, which corresponds to the beginning of a counterclockwise turn.
We concluded from these experiments that the tendency to turn counterclockwise is probably due in large part to visuospatial factors. By moving to the right to start a counterclockwise turning, people probably express a preference for keeping the zone of interest in the left visual field, controlled by the right hemisphere which is more involved in visual attention than the left. The relationship between this intrinsic factor and the cultural factors likely to influence the counterclockwise tendency is discussed.
